1. Field of the Invention
The present invention relates to electrochemical machining method and apparatus using ultrapure water, and more particularly to electrochemical machining method and apparatus which can perform machining for removal, formation of an oxide film, or film formation of a workpiece, such as a semiconductor material or a metallic material, with the use of only ultrapure water as an electrolytic solution.
2. Description of the Related Art
In recent years, the progress of technology has developed various new materials one after another. However, the machining technology useful for these new materials has not been established yet, and the development of the machining technology has followed the development of new materials.
Further, components in various types of equipments have become finer and have required higher accuracy. As submicronic manufacturing technology has commonly been used, the properties of materials are largely influenced by the machining method. Under these circumstances, in such a conventional machining method that a desired portion in a workpiece is physically destroyed and removed from the surface thereof by a tool, a large number of defects may be produced to deteriorate the properties of the workpiece. Therefore, it becomes important to perform machining without deteriorating the properties of the materials.
Some machining methods, such as chemical polishing, electrochemical machining, and electropolishing, have been developed in order to solve this problem. In contrast with the conventional physical machining, these methods perform machining for removal or the like by chemical elution. Therefore, in these methods, alteration of a machined layer due to plastic deformation and defects such as dislocation are not occurred, so that machining can be performed without deteriorating the properties of the materials.
Further, attention has been directed to a machining method utilizing interatomic chemical interaction. This method utilizes fine particles, radicals having highly chemical reactivity, and the like. According to this machining method, machining for removal or the like is performed by a chemical reaction with a workpiece on an atomic level. Therefore, machining can be controlled on an atomic level. This type of machining method includes elastic emission machining (EEM) and plasma chemical vaporization machining (CVM), which have been developed by the inventors. The EEM utilizes a chemical reaction between fine particles and a workpiece, and can realize machining on an atomic level without deteriorating the properties of materials. On the other hand, the plasma CVM utilizes a radical reaction between a workpiece and radicals produced in plasma at atmospheric pressure, and can realize machining on an atomic level.
In the aforementioned electrochemical machining and electropolishing, it is considered that machining proceeds through electrochemical interaction between a workpiece atom and an electrolyte ion in an electrolytic solution (an aqueous solution of NaCl, NaNo3, HF, HCl, HNO3, NaOH, or the like). The contamination of the workpiece with the electrolytic solution is unavoidable as long as the electrolytic solution is used.
Accordingly, the inventors have considered that, in neutral and alkaline electrolytic solutions, machining is related to hydroxide ions (OHxe2x88x92). This consideration has led to such an idea that machining can be performed even with water containing a small number of hydroxide ions. The inventors have experimentally confirmed the feasibility of this technology, and have proposed a method of increasing the ionic product in ultrapure water, in which a small number of unavoidable impurities is contained, as disclosed in Japanese laid-open Patent Publication No. 10-58236.
According to the above method, a workpiece is immersed in ultrapure water having an increased concentration of the hydroxide ion, and machining for removal or oxide film formation is performed through a chemical elution or an oxidation reaction with the hydroxide ions. Further, the inventors have also proposed the utilization of an electrochemical reaction on the surface of a solid having an ion exchange function or catalytic function to increase the number of the hydroxide ion. As a result, there has been developed a novel machining method which can realize machining in a clean manner without leaving impurities on the machined surface through the utilization of hydroxide ions in ultrapure water. This machining method has been expected to be used in wide applications including the field related with semiconductor manufacturing. Thus, the inventors have proposed a machining method using ultrapure water as an electrolytic solution, which is a low-damage machining method utilizing a chemical reaction, and a clean and low-environmental load machining method.
The present invention has been made in view of the above drawbacks. It is therefore a first object of the present invention to provide electrochemical machining method and apparatus which use ultrapure water as an electrolytic solution and can efficiently perform machining for removal of a material such as aluminum and iron.
It is a second object of the present invention to provide electrochemical machining method and apparatus which use ultrapure water as an electrolytic solution and, even in the case of a material such as aluminum and silicon, can realize not only machining for formation of an oxide film, but also machining for removal.
It is a third object of the present invention to provide electrochemical machining apparatus and method which can further improve accuracy of machining a workpiece.
In order to attain the first object, according to a first aspect of the present invention, there is provided an electrochemical machining method comprising: disposing a workpiece as an anode and a cathode in ultrapure water in such a state that a predetermined space is formed between the workpiece and the cathode; disposing a catalyst having an anion exchange function between the workpiece and the cathode; and relatively moving the workpiece and the catalyst while a voltage is applied between the workpiece and the cathode.
Thus, electrochemical machining is performed in ultrapure water in such a state that the workpiece serves as an anode, and a catalyst having an anion exchange function is used. Accordingly, machining for removal can efficiently be performed even with a material, such as aluminum and iron, which has been difficult to be machined for removal. Further, machining can stably be performed by increasing a flow rate of the ultrapure water flowing between the workpiece and the cathode as the counter electrode.
According to a preferred aspect of the present invention, the workpiece is selected from the group consisting of aluminum, iron, and copper.
According to a second aspect of the present invention, there is provided an electrochemical machining apparatus comprising: a machining chamber for holding ultrapure water; a cathode immersed in the ultrapure water held in the machining chamber; a workpiece holding portion for holding a workpiece at a predetermined distance from the cathode so that a surface, to be machined, of the workpiece is brought into contact with the ultrapure water; an anode contact brought into contact with the workpiece held by the workpiece holding portion so that the workpiece serves as an anode; a catalyst having an anion exchange function, the catalyst being disposed between the cathode and the workpiece held by the workpiece holding portion; a power source for applying a voltage between the cathode and the workpiece; and a moving mechanism for relatively moving the workpiece and the catalyst.
In order to attain the second object of the present invention, according to a third aspect of the present invention, there is provided an electrochemical machining method comprising: disposing a workpiece as a cathode and an anode in ultrapure water in such a state that a predetermined space is formed between the workpiece and the anode; disposing a catalyst for dissociating water molecules into hydrogen ions and hydroxide ions, between the workpiece and the anode; and applying a voltage between the workpiece and the anode.
Thus, electrochemical machining is performed in ultrapure water in such a state that the workpiece serves as a cathode. Accordingly, machining for removal can be realized even with a large of number materials, such as silicon or aluminum, which have been mainly machined for film formation and difficult to be machined for removal in the case where the workpiece serves as an anode.
According to a preferred aspect of the present invention, the workpiece and the anode are relatively moved while a voltage is applied between the workpiece and the anode.
Thus, the relative movement of the workpiece and the anode can increase the flow rate of the ultrapure water flowing between the workpiece and the anode as the counter electrode, so that stable electrochemical machining can be realized.
According to a fourth aspect of the present invention, there is provided an electrochemical machining apparatus comprising: a machining chamber for holding ultrapure water; an anode immersed in the ultrapure water held in the machining chamber; a workpiece holding portion for holding a workpiece at a predetermined distance from the anode so that a surface, to be machined, of the workpiece is brought into contact with the ultrapure water; a cathode contact brought into contact with the workpiece held by the workpiece holding portion so that the workpiece serves as a cathode; a catalyst disposed between the anode and the workpiece held by the workpiece holding portion for dissociating water molecules into hydrogen ions and hydroxide ions; and a power source for applying a voltage between the anode and the workpiece.
According to a preferred aspect of the present invention, the catalyst is a nonwoven fabric having an ion exchange function. This nonwoven fabric may be prepared in such a manner that graft chains are introduced into a nonwoven fabric having a proper fiber diameter and porosity with the use of the so-called radiation-induced graft polymerization, which comprises the gamma irradiation and the graft polymerization, for example. A cloth made of ion exchange fibers or a net into which an ion exchange group is introduced can be used as the catalyst member.
It is advantageous that the nonwoven fabric is brought into contact with the anode or the workpiece (cathode) from the viewpoint of increasing the current value. When the reaction products are likely to stay between the electrodes and the nonwoven fabric, and electrochemical machining may be performed unevenly over the machined surface, the electrochemical machining can be performed evenly over the machined surface by increasing the flow rate of the ultrapure water. In order to immediately remove the reaction products from the workpiece or the anode, a space may be formed between the nonwoven fabric and the electrodes, particularly between the nonwoven fabric and the workpiece.
According to another preferred aspect of the present invention, an ion exchange group in the nonwoven fabric having an ion exchange function is selected from the group consisting of anion exchange groups and cation exchange groups. The machining rate for removal and film formation can be controlled by changing the ion exchange fabric.
In order to attain the third object of the present invention, according to a fifth aspect of the present invention, there is provided an electrochemical machining apparatus comprising: a machining chamber for holding ultrapure water; a rotatable holding portion for detachably holding a workpiece so as to immerse the workpiece in the ultrapure water held in the machining chamber; a machining electrode immersed in the ultrapure water held in the machining chamber, the machining electrode being disposed at a predetermined distance from the workpiece held by the holding portion; a catalyst disposed between the workpiece held by the holding portion and the machining electrode for dissociating water molecules present between the machining electrode and the workpiece into hydrogen ions and hydroxide ions; a power source for applying a voltage between the machining electrode and the workpiece; and an ultrapure water supply nozzle for supplying ultrapure water between the machining electrode and the workpiece from the upstream side of a direction of rotation of the machining electrode.
With this arrangement, the surface roughness of the machined surface can be reduced. The reason for this is considered as follows: The ultrapure water is supplied between the machining electrode and the workpiece from the upstream side of the direction of rotation of the machining electrode, for thereby effectively removing gas bubbles and machining products, which has been difficult to be removed by simply rotation of the machining electrode, from the machined surface. Thus, the factors that prevent the flattening of the workpiece can be eliminated. Further, the deposition of fibers, which is a phenomenon specific to the use of a strongly acidic cation exchange fiber as the catalyst, can be prevented.
According to a sixth aspect of the present invention, there is provided an electrochemical machining apparatus comprising: a machining chamber for holding ultrapure water; a rotatable holding portion for detachably holding a workpiece so as to immerse the workpiece in the ultrapure water held in the machining chamber; a rotatable machining electrode immersed in the ultrapure water held in the machining chamber, the machining electrode being disposed at a predetermined distance from the workpiece held by the holding portion; a catalyst disposed between the workpiece held by the holding portion and the machining electrode for dissociating water molecules present between the machining electrode and the workpiece into hydrogen ions and hydroxide ions; and a power source for applying a voltage between the machining electrode and the workpiece; wherein the machining electrode has at least one of a columnar shape and a cylindrical shape, and a shaft center in a direction parallel to a plane of rotation of the workpiece held by the holding portion.
When only the machining electrode is rotated, machining traces in the direction of movement of the machining electrode are formed upon machining. With the above arrangement, the machining traces can be removed from the machined surface.
According to a seventh aspect of the present invention, there is provided an electrochemical machining apparatus comprising: a machining chamber for holding ultrapure water; a rotatable holding portion for detachably holding a workpiece so as to immerse the workpiece in the ultrapure water held in the machining chamber; a rotatable machining electrode immersed in the ultrapure water held in the machining chamber, the machining electrode being disposed at a predetermined distance from the workpiece held by the holding portion; a catalyst disposed between the workpiece held by the holding portion and the machining electrode for dissociating water molecules present between the machining electrode and the workpiece into hydrogen ions and hydroxide ions; and a power source for applying a voltage between the machining electrode and the workpiece; wherein the machining electrode has at least one of an elliptical shape and a spherical shape, and a shaft center in a direction parallel to a plane of rotation of the workpiece held by the holding portion.
With this arrangement, the surface roughness of the machined surface can be reduced. The reason for this is considered that the area of the machining portion is so small that the ultrapure water can easily be supplied to a portion around the machining portion to perform machining under stable conditions.
In these cases, the electrochemical machining apparatus should preferably comprise an ultrapure water circulation/purification device for purifying the ultrapure water held in the machining chamber to circulate the ultrapure water; and a high-pressure ultrapure water supply unit for supplying high-pressure ultrapure water to the ultrapure water supply nozzle.
According to an eighth aspect of the present invention, there is provided an electrochemical machining method comprising: disposing a machining electrode and a workpiece in ultrapure water in such a state that a predetermined space is formed between the machining electrode and the workpiece; disposing a catalyst for dissociating water molecules into hydrogen ions and hydroxide ions, between the machining electrode and the workpiece; applying a voltage between the machining electrode and the workpiece; and supplying ultrapure water between the machining electrode and the workpiece from the upstream side of a direction of rotation of at least one of the machining electrode and the workpiece while at least one of the machining electrode and the workpiece is being rotated.
According to a ninth aspect of the present invention, there is provided an electrochemical machining method comprising: disposing a machining electrode and a workpiece in ultrapure water in such a state that a predetermined space is formed between the machining electrode and the workpiece; disposing a catalyst for dissociating water molecules into hydrogen ions and hydroxide ions, between the machining electrode and the workpiece, applying a voltage between the machining electrode and the workpiece; and simultaneously rotating the machining electrode and the workpiece while the machining electrode and the workpiece are brought into line contact with each other.
According to a tenth aspect of the present invention, there is provided an electrochemical machining method comprising: disposing a machining electrode and a workpiece in ultrapure water in such a state that a predetermined space is formed between the machining electrode and the workpiece; disposing a catalyst for dissociating water molecules into hydrogen ions and hydroxide ions, between the machining electrode and the workpiece; applying a voltage between the machining electrode and the workpiece; and simultaneously rotating the machining electrode and the workpiece while the machining electrode and the workpiece are brought into point contact with each other.